Coursework Example of Civil Aircraft Nacelle Design

For coursework on the Bizjet, the following factors are taken from the turbofan class and type:

Uninstalled Tsls = 15.6 kn (3,500 lb)

Bare engine length = 1.547 m (5.07 ft)

Maximum diameter = 1.074 m (3.52 ft)

Engine airmass flow at takeoff = 66.2 kg/s (146 lb/s)

Engine airmass flow at maximum cruise rating « 20 kg/s (44 lb/s)

BPR = 4

Fan-face diameter = 0.716 m (2.35 ft)

This results in the following nacelle dimensions (factors are from industrial experience).

Intake Geometry (see Section 10.8.1)

• Highlight diameter = 0.9 x 0.716 m = 0.644 m (2.11 ft)

• Throat diameter (use the contraction ratio 1.12) = 0.575 m (1.89 ft), ATh = 0.26 m2 (2.8 ft2)

• Check air velocity at the throat for the maximum cruise condition at Mach 0.74

(716 ft/s) and at 41,000-ft altitude (pTO = 17,874 N/m2, TTO = 216.65 K, and =

0.284 kg/m3)

• Therefore, VTh = 20/(0.284 x 0.26) = 270 m/s (689 ft/s); i. e., Mach 0.922 is a preferred number

• This could result in low diffuser length: Ldiff = 0.65 x Dfan = 0.65 x 0.716 = 0.465 m (1.5 ft)

Lip Section (Crown Cut)

• Use (b + d) = 0.18 x Ldiff = 0.18 x 0.465 = 0.0837 m (0.275 ft)

• Use b = 1.4 x d; this results in b = 0.05 m (0.164 ft) and d = 0.0337 m (0.11 ft)

• Use a lower-lip fineness ratio of a/b = 2; this gives a = 0.1m (0.328 ft)

• Use an upper-lip fineness ratio of c/d = 4; this gives c = 0.134 m (0.44 ft)

• The intake length from the highlight (low-speed aircraft) = 0.465 + 0.1 = 0.565 m (1.85 ft)

• The Lfb is about 1 m from the highlight, which is 1.4 times Dfan = 1.4 x 0.716 = 1m(3.28ft)

• The crown-cut radius at LFB is 1.15 times Rfan = 1.1 x 0.716/2 = 0.41 m (1.34 ft) Lip Section (Keel Cut)

This book keeps the internal contour of the intake circular to fit with the rotating circular fan face; even the constrained Boeing 737 with a flat keel section must be circular at the fan face. Therefore, the internal contour of the keel cut is the mirror image of the crown cut about the centerline. However, the keel-cut radius at LFB is

1.4 times Rfan = 1.4 x 0.716/2 = 0.5m(1.64ft).

The intake internal contour can be finalized by taking the inflexion point at about mid Ldiff, maintaining maximum в within the range (i. e., 8 to 9 deg).

The average diameter at the maximum circumference = 0.41 + (0.5 – 0.41)/2 = 0.91m (3 ft).

Nozzle Geometry

Use a nozzle length = 0.75 x fan-face diameter = 0.75 x 0.716 m = 0.537 m (1.76ft). Once the control points for the geometry are established, the contour can be generated in CAD using splined curves (i. e., smooth fairing when drawn manually).

The total nacelle length = intake length + engine length + nozzle length = 0.565 + 1.547 + 0.537 = 2.65 m(« 8.7 ft), which is close to what was established in Chapter 6 from the statistical data.

The nacelle fineness ratio = 2.62/1.074 = 2.44 (i. e., within the range).

The author cautions that the empirical method presented from his industrial experience is coarse but nevertheless provides a representative geometry for the coursework exercise. This physical model serves as a starting point for further aero­dynamic refinement to a more slimline shape through CFD and testing. To obtain the factors used in the example, significant nacelle geometric data are required for better substantiation. (Readers must study many designs to get a sense of the factors used here.) Each industry has its own approach based on past designs (which form the basis of statistical data) to generate a nacelle geometry. In the industry, nacelle design is an involved process that includes the points addressed herein.

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